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(1) Field of the Invention
The invention relates to a method for illuminating particles contained in a medium for optical analysis, the medium being in contact with a window and subjected to a light beam that comes from a light source, through the window, and illuminates the particles to form an image of the particles; the window that is used being a trapezoidal prism a longer side surface of the parallel side surfaces of which forms a surface facing the medium; the light beam being modified to provide a beam of a predetermined thickness in the direction of the window normal; the light beam being directed to pass through one of the oblique surfaces of the prism serving as the window and to further travel inside the medium containing particles in the vicinity of the window surface making contact with the medium, substantially parallel to the surface, to thereby illuminate particles contained in a precisely predefined volume portion of the medium; and the image being formed by the light that is reflected back from the particles through the window. The invention also relates to a particle analyser.
The methods described above for illuminating particles, and analysers utilizing such methods are widely known in the industry. It is common in the industry today to transport or process material in the form of a particle suspension contained in a medium, such as a fluid. The particle referred to may be for example a crystal, fibre, grain, bubble, droplet, etc. The medium, in turn, may be water, or some other suitable, for example gaseous, substance.
Data about the amount of particles, their size distribution or shape is needed for monitoring and controlling different manufacturing processes. It is advantageous if such data can be brought into use timely and on a continuous basis. Therefore the best way to obtain the data would naturally be to receive it from an analyser that makes direct measurements from a process tube or container. This would ascertain that the measured sample provides an accurate picture of the process.
An optical measurement principle can be used to produce a particle analyser in which particles are illuminated and the light reflected by them is measured by an optical detector. The optical detector is selected according to the analysing task to be performed. The optical detector may thus be anything from a basic light cell to a multi-element image analyser. The reflected light can also be examined with the naked eye or by using suitable auxiliary optics. An essential aspect is that the particles to be illuminated are in a known, precisely defined volume portion of the medium. By restricting the illumination of the particles to a predetermined volume portion, a good contrast is obtained for the image received by the detector. As regards quantitative analysis, knowing the size of the fluid volume containing the illuminated particles provides a significant advantage.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
One of the prior art solutions that can be mentioned is the crystal microscope, which is commonly used in sugar industry, one example of which is Jungner Crystal Projector, type KP3. This device has two windows between which the suspension to be measured travels. The windows form two parallel planes that are at an adjustable distance from one another. Particles, for example sugar crystals, are illuminated through one of the windows and an image of the particles is formed by means of a lens that is behind the other window. The image is projected in an enlarged form onto a matt-finished glass plate, thus enabling the personnel to examine the size and shape of the particles. The contrast is poor because the illuminated particles are visible against the light and therefore a precise automated electronic image analysis cannot be applied. It is not possible to draw quantitative conclusions either because the flow in the gap between the windows does not provide a correct picture of the main flow, i.e. the sample is not representative enough.
Another extremely wide group of prior art devices consists of what are known as photometers, which measure light absorption or reflection caused by particles. A typical example of a photometer that can be mentioned is the device described in U.S. Pat. No. 3,962,581. A characteristic of a photometer-type device is that the illumination is provided by a cone of rays passing relatively perpendicularly through a process window. A disadvantage is that the borders of the illuminated volume portion are not precisely marked and therefore the device is only applicable in quantitative determining based on an adjustment curve. The adjustment curve must always be experimentally defined for each application separately.
To eliminate the shortcomings in the solutions of the prior art described above, a method for illuminating particles for optical analysis and an optical particle analyser were developed in the past, the analyser being described in the following patent publications: FI 77330 (U.S. Pat. No. 4,776,697; JP 1,681,781; DE 3,700,286). A shortcoming of this prior art solution involves the changing of the angle of the light beam used in the illumination. Because the device is to be used for measuring particles in different media having different refractive indexes, the angle of the light beam used in the illumination must be changed according to the refractive index concerned. The adjustment of the illumination angle need not take place on a continuous basis, but it must be possible to set the angle application-specifically, i.e. whenever the refractive index of the medium to be measured changes. Another disadvantage is that the angle formed by the direction from which the particles are viewed and the direction of the illumination is great and therefore the analyser easily becomes impractical in terms of size and shape. These disadvantages are not discussed in any way in the Finnish Patent Publication, i.e. the publication does not even put forward any suggestions as to how to solve these problems.
It is an object of the present invention to provide a method and an optical particle analyser that allow the shortcomings of the prior art to be eliminated. This is achieved by the invention. The method of the invention is characterized in that the light beam coming from the light source is directed to pass in the vicinity of the prism serving as the window through a mirror surface and a cylindrical lens, which direct the light beam to the prism and convert a shift of the light source to a change in the light beam angle. The optical particle analyser of the invention, in turn, is characterized in that the particle analyser comprises a mirror surface and a cylindrical lens placed in the vicinity of a prism serving as the window, the surface and lens being configured to direct a light beam coming from a light source to the prism and convert a shift of the light source to a change in the light beam angle.
One of the major advantages of the invention is that the light beam angle used in the illumination can be changed in a very simple manner according to the refractive index of the medium. A further advantage is that when seen from the light source, the particles are viewed and illuminated from the same direction and thus the analyser can be made practical in terms of size and shape. Still another advantage of the invention is that it is simple and therefore economical to implement.